Register.php

 public function setServiceManager(ServiceManager $serviceManager)
    {
        $this->get('user')
             ->setHydrator($serviceManager->get('entityHydrator'));

        $filter = $this->getInputFilter();

        // We need to add a few filters
        $email = $filter->get('user')
                        ->get('email');


        $cb = new \Zend\Validator\Callback(array('callback' => function() {}));

        ob_clean();
        var_dump($email->getValidatorChain()->addValidator($cb));
        exit;
    }